Due to the shrinking available capacity in landfills for the disposal of solid waste, a corresponding reduction in the volume of municipal solid waste for such disposal has been pursued. The principal method used in this program is the incineration of combustible materials. Although such a program has shown to be successful in reducing the volume of municipal solid waste, as well as having the added advantage of producing energy, exhaust emissions from these plants need to be rigidly controlled so as to minimize the amount of carbon monoxide and unburned hydrocarbons emitted. Some states have set stringent requirements as to the amount of carbon monoxide allowed in exhaust emissions as well as a minimum oxygen level. Failure to meet these emission requirements could result in the shut down of the incinerator. By operating these incinerators more efficiently, more complete combustion will result and hence, exhaust emissions will satisfy the statutory requirements.
One type of incineration plant is known as a water-cooled rotary combustor. An example of such a combustor is described in U.S. Pat. No. 3,822,651 to Harris et al., which is incorporated herein by reference. A water-cooled rotary combustor generally includes a combustion barrel having a generally cylindrical side wall affixed to annular support bands which are received on rollers to permit rotation of the barrel about its longitudinal axis. The barrel has a generally open input end for receiving material to be burned, such as municipal solid waste which can vary in moisture content. The opposite or output end of the barrel is disposed in a flue. The combustion barrel is tilted from the horizontal, the input end being higher than the output end. As the waste material burns, it travels along the longitudinal axis of the barrel such that solid combustion products exit the barrel at the lower output end. Exhaust gases and solid combustion products exit the barrel at the output end. The combustion barrel is cooled by cooling pipes joined by gas porous interconnections to form the generally cylindrical side wall of the barrel.
Since the composition of the waste material varies, it can be difficult to maintain a constant feed rate of the solid waste into the barrel, and thus the intensity of the fire varies over time. Also, the heat of combustion of solid waste for each input charge into the combustor varies greatly. As a result, the constitution of the exhaust gases can also vary over time. By controlling the rate of combustion within the barrel, a more efficient incineration occurs and produces a more stable constitution of the exhaust gases and less unburned hydrocarbons. More particularly, it is important to maintain the carbon monoxide level below 100 ppm since that is the level required by most State laws. Another requirement imposed on the operators of municipal waste incinerators is that the oxygen level in the exhaust gases not fall below 3%.
A method for automatic combustion control for a rotary combustor is described in pending application Ser. No. 018,682, filed on Feb. 25, 1987, of which applicant is a co-inventor and which is assigned to the present assignee. An oxygen sensor located within the flue is used to detect the percentage of oxygen present in the exhaust gases. The output from this sensor is used to control the amount of combustion gas supplied to the combustion barrel in order to maintain a percentage of oxygen in the flue gas near a predetermined level. In addition, flame and temperature sensors, which can be photoelectric cells and infrared sensors, can be used to detect temperature and the existence of a flame in an area above each of the windboxes, so that combustion gas supplied to each windbox can be individually controlled.
Another problem associated with inefficient combustion of municipal solid waste within a rotary combustor is that of clinker formation. Clinkers, usually consisting of molten ash, softened glass material, etc., can be formed in the combustor and can cause problems in combustor performance. The major cause for the clinker to form is a localized hot spot in the combustor. Due to the varied nature of municipal solid waste, it is not always possible to have a perfectly uniform and even burning fuel bed in the combustor.
Thus, the object of the present invention is to most accurately control the amount of carbon monoxide and unburned hydrocarbons present in the exhaust of a rotary combustor so as to provide for the most efficient combustion of municipal solid waste.
Another object of the present invention is to compensate for changes in the rate of combustion occurring within a rotary combustor due to the variable nature of municipal solid waste.
It is a further object of the present invention to automatically control the combustion rate in a rotary combustor so as to maintain the temperature in the combustor at a stable level which is high enough to complete combustion, but at a level below where the clinker starts to form.